Biaxially oriented polyester film

ABSTRACT

According to the present invention, a biaxially oriented polyester film is provided which comprises at least one film layer composed predominantly of polypropylene terephthalate and has a heat shrinkage of 0.8% or below after standing at 80° C. for 30 minutes. This film is excellent in wear resistance and hence useful particularly for magnetic recording media.

This application is the national phase under 35 U.S.C. §371 of PCTInternational Application No. PCT/JP98/02590 which has an Internationalfiling date of Jun. 12, 1998, which designated the United States ofAmerica.

TECHNICAL FIELD

This invention is directed to biaxially oriented polyester films.

BACKGROUND ART

As biaxially oriented polyester films, a biaxially oriented laminatedpolyester film is known (for instance, U.S. Pat. Nos. 5,069,962 and5,626,942). There is also known a biaxially oriented polypropyleneterephthalate film (Japanese Unexamined Patent Publication No. 9-175055for example).

Such a commonly known, biaxially oriented polyester film when in use formagnetic recording media affords improved electromagnetic conversioncharacteristics, but leaves the problem that polymer particles becomeescaped due to insufficient wear resistance of the polymer surface,eventually inviting particle dusting. Upon application to magnetictapes, this type of polyester film involves the lack of signals whichwould result from particle dusting. In magnetic recording media of ahigher density, a need exists for those physical characteristics thatcould prevent polymer particles from escaping out of the correspondingpolyester film. In order to solve these problems, a principal object ofthe present invention is to provide a biaxially oriented polyester filmwhich, in particular, is excellent in wear resistance and free fromoligomer separation.

DISCLOSURE OF THE INVENTION

The biaxially oriented polyester film according to the present inventionis so constituted as to have at least one film layer disposed, whichfilm layer is composed predominantly of polypropylene terephthalate. Afirst embodiment of the polyester film lies in such having a heatshrinkage of 0.8% or below after standing at 80° C. for 30 minutes. Asecond embodiment of the polyester film lies in such having on at leastone surface a surface roughness Ra of 5-120 nm, a 10-point averageroughness Rz/Ra of 12 or below and a protrusion-to-protrusion spacing Smof 15 μm or below.

BEST MODE OF CARRYING OUT THE INVENTION

To gain high resistance to wear and freedom from oligomer separation,the polypropylene terephthalate (hereinafter called PPT) for use in thepresent invention is derived preferably by polymerization of1,3-propanediol with terephthalic acid, or a methyl ester derivative orthe like thereof. A blend of two or more different polymers or acopolymer is also suitably useful so long as it has no adverse effectson achieving the object of the invention.

The PPT-predominated film layer used herein is one in which a PPTcomponent is contained in an amount of more than 50% by weight.

The PPT-predominated film layer (hereinafter called the layer A whererelevant) may be incorporated with an inorganic particle material suchas of aluminum silicate, calcium carbonate, alumina, silica, calciumphosphate, titanium oxide or the like, or with an organic particlematerial so that the film layer is made resistant to wear. The averageparticle diameter of such particle material is in the range of 0.01-2.0μm, preferably of 0.02-1.5 μm, more preferably of 0.02-1.0 μm. Further,the relative standard deviation of this particle diameter is preferably0.5 or below, more preferably 0.3 or below, most preferably 0.2 orbelow. The content of that particle material is in the range of 0.01-3%by weight, preferably of 0.02-2% by weight, more preferably of 0.05-1%by weight. The layer A may be incorporated with various additives suchas an antioxidant, a heat stabilizer, an ultraviolet absorber and thelike in conventional amounts, provided that the object of the inventionis not adversely affected.

The biaxially oriented polyester film provided by the present inventionmay be a single-layered film formed solely of the above-mentionedPPT-predominated film layer.

In the case where the biaxially oriented polyester film provided by thepresent invention is of a laminated structure in which two or morelayers are superposed one on another, at least one of the constituentfilm layers should be formed of the PPT-predominated film layerdescribed above. Though not particularly restricted, the other filmlayer or layers may preferably be formed of polyethylene terephthalate(hereinafter called PET), poly(ethylene-2,6-naphthalene dicarboxylate)(PEN) and the like. A blend of two or more different polymers or acopolymer may be used unless the object of the invention is adverselyaffected. These film layers can be incorporated with the same inorganicor organic particle material as noted above in connection with the layerA. Such additives as an antioxidant, a heat stabilizer, an ultravioletabsorber and the like may also be added in conventional amounts unlessthe object of the invention is adversely affected.

According to the first embodiment of the present invention, a biaxiallyoriented polyester film should have a heat shrinkage of 0.8% or belowafter standing for 30 minutes at 80° C. so as to prevent signals fromgetting lacked when in use for magnetic recording media. The heatshrinkage is preferably 0.6% or below, more preferably 0.4% or below.

The thickness of the layer A is not particularly restricted.

Desirably, however, it may be set to be in the range of 0.01-3.0 μm,preferably of 0.02-2.0 μm, more preferably of 0.03-1.0 μm, in respect ofthe increase in wear resistance and the preclusion of oligomerseparation.

No particular restriction is imposed upon the relationship between thethickness t of the layer A and the average particle diameter d of theparticle material contained in the layer A. However, the wear resistancecan noticeably be improved in the case of 0.2 d≦t≦10 d, preferably of0.3 d≦t≦5 d, more preferably of 0.5 d≦t≦3 d. When two or more layers Aare used and when two such layers are disposed for example as theoutermost front and back surfaces of the finished film, those equationsshould preferably be satisfied.

According to the second embodiment of the present invention, a biaxiallyoriented polyester film should have on at least one surface a surfaceroughness Ra of 5-120 nm, a 10-point average roughness Rz/Ra of 12 orbelow and a protrusion-to-protrusion spacing Sm of 12 μm. With wearresistance in view, the surface roughness Ra is preferably in the rangeof 5-50 nm, especially of 10-30 nm, the 10-point average roughness Rz/Rais preferably 10 or below, and the protrusion-to-protrusion spacing Smis preferably 12 μm or below. The lower limit of Rz/Ra is notparticularly restrictive which, however, is 4 or above for practicalfilm production, and the lower limit of Sm is not particularlyrestrictive which, however, is in the order of 3 for practical filmproduction.

Also in the second embodiment, it is desired that the relationshipbetween the thickness t of the layer A and the average particle diameterd of the particle material contained in the layer A be observed in thesame ranges as specified above in connection with the first embodiment.

In the present invention, a further embodiment is included which isdesigned to simultaneously comply with the requirements of the first andsecond embodiments. In addition and more advantageously, each of thefirst and second embodiments should meet the following requirements.

To increase wear resistance, to render oligomer separation free and todefine surface profiling effectively, each of the biaxially orientedpolyester films according to the present invention is preferably broughtinto a multi-layered structure in which at least two or more film layersare placed in superposed relation to one another. The crystallizationparameter ΔTcg of an outermost layer-constituting polymer should bepreferably lower than 60° C., more preferably lower than 50° C.,especially lower than 40° C., in view of wear resistance and dimensionalstability. The crystallization parameter ΔTcg is defined by thedifference between the cold crystallization temperature during thecourse of temperature rise and the glass transition temperature. Thesmaller difference, the speed of polymer crystallization becomes higherwith consequent arrival at a specific range of thermal shrinkage afterstanding for 30 minutes at 80° C. that falls within the scope of thepresent invention. This means that those features contemplated under theinvention can also be effectively attained.

In the biaxially oriented polyester film of the present invention, theratio of X/Y is set to be usually 5 or above from the points of view ofwear resistance and traveling capability, wherein ( X) is the number ofprotrusions defined on the surface of the layer A, and (Y) is the numberof particles contained in the layer A. The ratio of X/Y is preferably 10or above, more preferably 50 or above. In the invention, surfaceprotrusions may be formed from a given particle to be added to the film,or without reliance on that particle, but from a multiplicity of finecrystals of the layer A that are derived by crystallization of a layerA-constituting polymer. In such instance, the ratio of protrusion innumber to particle in number might presumably be extremely high inprinciple as the number of particles is small, but the upper limit isaround 100 tenths of thousands.

The biaxially oriented polyester film of the present invention shoulddesirably have a Young's modulus of 4.5 GPa or above, especially 5 GPaor above, in the lengthwise and widthwise directions. The Young'smodulus may be identical or different in the two directions. Forexample, in the case of use for magnetic recording media, insufficientmodulus in the base film causes the resultant magnetic tape stretchableduring traveling thereof under the influence of a tension arising from amagnetic head or a guide pin, consequently producing inadequate effectson the electromagnetic conversion characteristics (outputcharacteristics). In magnetic recording tapes for long-playing use,improved modulus is preferable at least in either one direction sincethe base film is generally small in thickness.

Moreover, the ratio of Young's modulus in a lengthwise direction to thatin a widthwise direction is preferably in the range of 0.7-1.5, morepreferably of 0.75-1.3, especially of 0.8-1.2. Particularly in magnetictapes using a helical scanning recording head, departures from theseranges lead to irregular contact of the tape with the head, resulting inunacceptable electromagnetic conversion characteristics.

Reference is further made to the equation of [0.08E−S] which is directedto the relationship between the modulus E (GPa) in a lengthwisedirection and the thermal shrinkage S (%) in a lengthwise directionafter standing for 30 minutes under the temperature conditions of 80° C.When this equation is set to satisfy 0.08 or above, also 0.09 or above,especially 0.1 or above, heat dimensional stability can be improvedwithout modulus reduced to so an appreciable extent.

In addition, from the viewpoint of dimensional stability, a polyestercomposition is preferred which is made up chiefly of 60-99.9% by weightof PPT and 40-0.1% by weight of PET.

Another similar composition is also preferred which is made of 80-99.5%by weight of PPT and 20-0.5% by weight of PET, especially of 90-99.9% byweight of PPT and 10-0.1% by weight of PET. In such cases, PET is set tohave an intrinsic viscosity (hereinafter called IV) of 0.6 or above,preferably of 0.65 or above. This viscosity requirement makes iteffective to bring about those wear resistance, dimensional stabilityand surface profiling properties that should accrue inherently from thebiaxially oriented polyester film of the present invention.

In the biaxially oriented polyester film of the present invention, theoverall film thickness is not particularly limited. When used as asubstrate for magnetic disks, however, such polyester film is formedwith an overall thickness of 50-100 μm, also of 50-80 μm, especially of60-80 μm so that good wear resistance can be obtained.

In regard to one surface layer of the biaxially oriented polyester filmaccording to the present invention, the surface roughness Ra and thesurface protrusion-to-protrusion spacing Sm are as described above inconnection with the second embodiment. In further regard to the other oropposite surface layer, it is desired that the surface roughness Ra beset to be 9 nm or below, preferably 6 nm or below, and that the surfaceprotrusion-to-protrusion spacing Sm be set to be 15 μm or below,preferably 10 μm or below. By strict observance of the two surfacelayers to fall within the ranges specified above, traveling capabilityand output characteristics can be well balanced on a high level when theresulting polyester film is applied to magnetic recording media,particularly to magnetic tapes of a digital recording system.

In the biaxially oriented polyester film according to the presentinvention, a laminated structure (C/A/B) of at least three layers ispreferred in which a polymer layer (layer A) predominantly of PPT is putin place on at least one surface of a polymer layer (layer C) mainly ofa thermoplastic resin C, and a polymer layer (layer B) mainly of athermoplastic resin B is disposed over at least one surface of the layerA. A four-layered laminate of A/C/A/B is more preferred. A five-layeredlaminate of B/A/C/A/B is most preferred in respect of abrasionresistance and output characteristics.

As the thermoplastic resin C used herein, a polyester is desired, butwithout limitation thereto. Suitable sorts. of polyester are typified bythose having contained as a chief component at least one recurring unitselected from ethylene terephthalate, ethyleneα,β-bis(2-chlorophenoxy)ethane-4,4′-dicarboxylate and ethylene2,6-naphthalate. To enhance mechanical strength and dimensionalstability, polyesters are preferred which are composed chiefly ofethylene terephthalate or ethylene 2,6-naphthalate. Particularlypreferred among these polyesters is such composed chiefly of ethyleneterephthalate that can be well laminated with the PPT-predominatedpolymer layer (A). A blend of two or more polyesters or a copolymer isacceptable on condition that the object of the present invention is notadversely affected. Additionally, a recycling polymer may be used wheredesired. By the recycling polymer is meant a polyester which has anamount of not less than 55 equivalent weight/10⁶ g of carboxylic acidbonded to the terminal and has a haze of not more than 20% as determinedfrom the polymeric solution. A particle material may be present orabsent in this polyester.

The thickness of the layer C is varied in respect of for what thecorresponding polyester film is used, and hence, is not particularlyrestricted. With mechanical strength in view, this layer preferably hasa thickness of larger than 50% of the overall film thickness. Largerthan 60% is more preferable, and larger than 70% is still morepreferable.

PPT used herein is as described hereinabove and may be obtained by anysuitable mode of polymerization known in the art. The polymer layer (A)is disposed on at least one surface of the polymer layer (C) chiefly ofa thermoplastic resin C. Preferably, upon arrangement of the polymerlayer (A) on both of the surfaces of the polymer layer (C), a biaxiallyoriented laminated polyester film is attainable which has a separatedoligomer markedly reduced in amount and has fine protrusions defined onthe surface.

The polymer layer (A) used herein is formed from a PPT polymer with anIV of preferably 0.8 or below, more preferably 0.9 or above. The upperlimit of IV is not particularly restrictive, but is usually at about 2.0or below such that lamination is possible with sufficient uniformity andsmall thickness. The above IV values of PPT ensure not only thin uniformlamination of the polymer layer (A) with the polymer layer (C) chieflyof a thermoplastic resin C, but also elimination of such defects asoligomer separation, abresion resistance and the like.

As the thermoplastic resin B used herein, polyester is desired thoughnot limited thereto. Suitable sorts of polyester are typified by thosehaving contained as a main component at least one recurring unitselected from ethylene terephthalate, ethyleneα,β-bis(2-chlorophenoxy)ethane-4,4′-dicarboxylate and ethylene2,6-naphthalate. To improve mechanical strength and dimensionalstability, polyesters are preferred which are composed of ethyleneterephthalate or ethylene 2,6-naphthalate as a main component.Particularly preferred is a polyester composed of ethylene terephthalateas a main component since it is suited for formation of a film laminatedwith the polymer layer of PPT.

The polymer layer (B) is substantially free of a particle material fromthe viewpoint of abrasion resistance. However, this polymer layer maycontain a particle material in an amount of less than 0.5% by weight solong as the particle has a particle diameter of less than 0.6 μm,preferably of 0.1 μm or below. When wear resistance is taken in view,suitable particles are chosen from aluminum silicate, alumina, silicaand the like, but without limitation thereto. These particles may beused in combination.

The thickness (Ta) of the polymer layer (A) is preferably less than 1μm, more preferably less than 0.8 μm, especially less than 0.5 μm. Morethan 1 μm in thickness in the polymer layer (A) invites impairedstretchability when in the production of a biaxially oriented laminatedpolyester film, thus causing stretch-broken film.

The ratio of thickness (Ta) of the polymer layer (A) to thickness (Tb)of the polymer layer (B) is set to meet the following requirements.

0.01≦Tb/Ta<1

More preferably,

0.03≦Tb/Ta<0.5

Especially preferably,

0.1≦Tb/Ta<0.3

This thickness ratio of the two polymer layers permits those surfaceprotrusions resulting from the PPT crystals on the surface of thepolymer layer (A) to define (trace) on the surface layer of the polymerlayer (B) having an extremely small thickness. Larger thickness ratiosof the polymer layers than the specified range fail to trace the PPTcrystal-induced protrusions of the polymer layer (A) on to the surfaceof the polymer layer (B). This is responsible for increased frictioncoefficient andhence for reduced traveling capability and diminishedabrasion resistance.

Conversely, smaller thickness ratios of the polymer layers than thespecified range make it impossible to uniformly laminate the polymerlayer (B), inviting broken laminate (not laminated in part) and hencespotted laminate. This renders the finished laminated film lessresistant to rubbing, and moreover, allows PPT crystal-inducedprotrusions to appear on the surface of the polymer layer (B). Theprotrusions in turn make the corresponding film tacky during running ofthe latter on a heating roll at a stage of film formation, producingpoor surfacing. The thickness of the polymer layer (B) to be laminatedis not particularly limited so long as it is set to be inside the rangespecified above. From the viewpoints of traveling capability, abrasionresistance and oligomer preclusion, 0.5 μm or below is preferable. Wherethe outermost two, front and back, surface layers of the finishedlaminated film are formed of the polymer layer (B), these surface layersmay be set to be of the same or different thickness. The surfaceroughness of the layer B can be controlled as desired upon adjustment ofthe thickness of such polymer layer to be laminated.

The polymer layer (B) has a multiplicity of fine protrusions defined onthe surface. The protrusions on the surface of the layer B should bederivable, in view of abrasion resistance, preferably from the PPTcrystals of the polymer layer (A). Addition of a particle materialcauses objectionable voids to take place. In the film of the presentinvention, surface protrusions are defined by those crystals depositedfrom a polymer itself of the layer A with the result that the voids canbe decreased to a great extent. Thus, these protrusions are lesssusceptible to breakage so that wear resistance can be improved withrubbing dust, drop out and the like alleviated.

The biaxially oriented polyester film of the present invention will findmany applications, for example, to magnetic recording media, packagingmaterials, and cards such as prepaid cards. This polyester film is alsosuitably applicable as a biaxially oriented polyester film for use indigital videotapes in which higher output is required, and is furtheruseful for data storage in computers and the like.

In the biaxially oriented laminated polyester film of the presentinvention, an adhesion-facilitating layer or adhesive layer may bedisposed on either one or both of the outer surface thereof. Resinsuseful in the adhesive layer are chosen, though not limited, frompolyester resin, acrylic resin, polyurethane resin and the like, butwithout limitation thereto, which are highly adhesive property to alayer composed predominantly of PPT.

The polyester resin used herein has an ester bond attached to the mainchain or side chain thereof. This resin is derivable frompolycondensation of an acid component with a glycol component.

When being used in the form of a coating liquid, the polyester resin maypreferably be copolymerized with a compound containing a basic group ofsulfonic acid or a compound containing a basic group of carboxylic acidso that the polyester resin is made highly adhesive to various paintsand inks, or is made easily soluble in water.

As the polyester resin, a modified polyester copolymer can also be usedwhich includes a block copolymer and a graft copolymer, both beingderived by modification with acrylic, urethane, epoxy or the like.

As the acrylic resin used herein, a modified acrylic copolymer is suitedwhich includes a block copolymer or a graft copolymer, both beingderived by modification with polyester, urethane, epoxy or the like.

As the polyurethane resin used herein, such a resin as being structuredto have a urethane bond in the molecule is suited but with no limitationplaced thereon. This resin is basically constituted of a reactionproduct obtained from a polyol compound and an isocyanate compound andmay be incorporated with a chain extender and the like where needed.

The chain extender used herein is chosen from ethylene glycol,diethylene glycol, propylene glycol, trimethylolpropane, hydrazine,ethylenediamine, diethylenetriamine and the like.

The adhesive layer may be incorporated with other different resins suchas epoxy resin, silicone resin, urea resin, phenol resin and the like,but to an extent not to impair the beneficial effects of the presentinvention. There may also be added various additives such as, forexample, an antioxidant, a heat stabilizer, a weathering agent, anultraviolet absorber, a lubricant, a pigment, a dye, an organic orinorganic fine particle, a filler, an antistatic agent, a nucleatingagent and the like.

Although the particle materials and crosslinking agents are added atwill to the adhesive layer, these additives contribute to improvementsin lubrication, blocking resistance, and adhesion to various paints andinks.

The particle material optionally incorporated in the adhesive layer ischosen from silica, colloidal silica, alumina, alumina sol, kaolin,talc, mica, calcium carbonate and the like, but without limitationthereto. The average particle diameter of the particle material, thoughnot particularly limited, is preferably in the range of 0.01-5 μm, morepreferably of 0.05-3 μm, most preferably of 0.08-2 μm. The mixing ratioof particle material to all resins in the adhesive layer is set to bepreferably in the range of 0.05-8 parts by weight, more preferably of0.1-3 parts by weight, as determined by the weight of solid contents,but the ratio specified here should not be construed as restrictive.

The crosslinking agent optionally incorporated in the adhesive layer ischosen from urea resin, melamine resin, acrylamide resin and polyamideresin, all being methylolated or alkylolated, an epoxy compound, anisocyanate compound, an oxazoline compound, an aziridine compound,various silane coupling agents, various titanate coupling agents and thelike, but without limitation thereto. The amount of the crosslinkingagent to be added, though not particularly limited, preferably in therange of 0.5-20 parts by weight, more preferably of 1-15 parts byweight, most preferably of 2-10 parts by weight, based on the weight ofall the resins contained in the adhesive layer.

The resins used for formation of the adhesive layer may be used afterbeing dissolved or dispersed in organic solvent or water. In particular,in view of economy, uniformity and adhesion to a substrate, the adhesivepolyester film used herein can preferably be obtained by means of inlinecoating that is effected at one process step in the production line ofpolyester films. Preferred, therefore, are dissolved or dispersedresins.

The thickness of the adhesive layer is set preferably in the range of0.02-5 μm, more preferably of 0.03-2 μm, most preferably of 0.05-0.5 μm,which should not be considered restrictive. Too small a thickness in theadhesive layer sometimes poses unacceptable bondability to variouspaints and inks.

One preferred form of a process for the production of the biaxiallyoriented polyester film of the present invention will now be illustratedand described. This process, however, should not be construed aslimiting to the invention.

Firstly, a particle material is caused to be contained in afilm-constituting PPT polymer. To this end, though not restricted, theremay be illustrated a method in which a propanediol slurry as a particlematerial is polymerized with an acid component such as terephthalicacid, or a method in which an aqueous slurry of a particle material ismixed and kneaded with a selected shape of PPT pellets with a biaxialkneading extruder of a vented type.

The content of the particle material may effectively be adjusted bypreparing a master of high concentration with use of one of the abovemethod, followed by dilution of the master with a substantiallyparticle-free polymer during film formation.

The resultant polymer pellets are then dried and supplied to a meltextruder where the pellet is extruded from a slit die into the shape ofa sheet. This sheet is cooled and solidified on a casting roll, wherebyan unoriented film is prepared. At this stage, the melted polyester isbrought into laminated condition by use of a plurality of extruders, aplurality of manifolds or an intermixing block.

Subsequently, the unoriented film is biaxially stretched and biaxiallyoriented. Stretching may be carried out by means of successive biaxialstretching or simultaneous biaxial stretching. More effectively, alengthwise direction and a widthwise direction are subjected in thatorder to successive biaxial stretching. Immediately before lengthwisestretching, heat treatment is done at a temperature of 60° C.-150° C.for a length of time of 1 second-20 seconds, and this heat treatment isgreatly conducive to the heat shrinkage and characteristic aspectsinherent to the present invention. Lengthwise stretching at threeseparate stages is effective for attaining the heat shrinkage accordingto the invention. The lengthwise stretching temperature set at 50-180°C., the lengthwise to widthwise stretching factor at 2.5-6.0 times, andthe lengthwise to widthwise stretching speed at 5,000-50,000%/minute canbe illustrated as preferred. To gain the characteristic aspectsaccording to the invention, the stretching speed is set at 20000%/minutein particular. Widthwise stretching is conducted by use of a tenter at astretching temperature of 50-180° C., at a widthwise stretching factorof 3.0-6.5 times that is set to be larger than a lengthwise stretchingfactor, and at a widthwise stretching speed of 1,000-20,000%/minute.When it is found necessary, restretching may further be done in thelengthwise and widthwise directions. Lengthwise restretching is effectedat 50-180° C. and at a stretching factor of 1.1-2.0 times, whereaswidthwise restretching is effected by use of a tenter at a stretchingtemperature of 50-180° C. and at a widthwise stretching factor of1.1-2.0.

Then, the biaxially oriented film obtained above is heat-treated underconstant tension. Heat treatment is effected at a temperature of120-250° C., particularly of 150-230° C., and for a length of time of0.5-60 seconds. Upon completion of the heat treatment, the resultantfilm is introduced in an intermediate cooling zone where it is slowlycooled at an intermediate cooling temperature of 60° C.-150° C. for 1second-60seconds. This intermediate cooling effectively leads to theheat shrinkage according to the present invention. Where either one ofthe lengthwise and widthwise directions is restretched, heat treatmentis once again conducted, subsequently to the intermediate cooling, at arelaxation ratio of less than 3% and at a temperature of 60° C.-130° C.for a period of time of 0.5-60 seconds so that heat shrinkage andYoung's moduli can be well balanced as desired in the invention.

[Measurement Methods of Physical Properties and Evaluation Methods ofBeneficial Effects]

Those characteristic values and beneficial effects exhibited by thepresent invention are determined in accordance with the followingmeasurement methods and the following evaluation methods.

(1) Average Particle Diameter of Particles and Number of Particles (Y)

A polyester is removed from a test film by means of plasma ashing,whereby particles are caused to expose from external view. Plasma ashingconditions are so selected that despite the polymer ashed, the particlescan be protected almost completely from being impaired. Observation ismade of the particles by a scanning electron microscope (SEM), and theresultant particle images are treated by an image analyzer. Themagnification of SEM is set to be approximately 2000-10000 times, andthe field in single measurement is chosen from about 10-50 μm in oneside. In terms of more than 5000 particles in number observed at variedlocations, the volume-average diameter d is determined by the particlediameter and volume fraction and by the following equation.

d=Σdi·Nvi

where di denotes the particle diameter, and Nvi denotes the volumefraction.

When the particles are of an organic nature and are apt to becomegreatly impaired due to plasma ashing at low temperature, the followingmethod may be employed.

The film is observed in cross section by a transmission electronmicroscope at a magnification of 3000-100000 times. The thickness of aslice for TEM inspection is set at about 100 nm and measured at a fieldof more than 500 at varied locations. The volume-average particlediameter d is obtained from the above equation.

(2) Number of Protrusions (X) and Ratio of Crystal-Induced Protrusions

A film is observed in cross section by a transmission electronmicroscope and at a magnification of 3000-200000 times. The thickness ofa slice for TEM inspection is set at about 100 nm and measured at afield of more than 500 at varied locations. Both the number ofprotrusions in all and the number of protrusions induced from particlesare counted, from which the ratio of protrusions induced from crystalsis determined.

Alternatively, the film is etched with use of a suitable solvent in thedirection of thickness and under the protrusions to be checked. Wheninsoluble matter remains as such having defined the protrusions, it istaken as a particle material having been extraneously added orinternally separated (I). When insoluble matter is absent or negligiblysmall if present, the protrusions are presumed to have been derived fromfine crystals (II). As the above solvent, a good example is a mixedsolvent of phenol/carbon tetrachloride (weight ratio: 6/4). In this wayand with a field of 1 mm² set, the frequencies of I and II aredetermined. The value of II/(I+II) may be used as the ratio ofcrystal-induced protrusions. Here, I+II is expressed as the number ofprotrusions X.

(3) Content of Particles

Compositional analysis is made by means of the microscopic FT-IR method(Fourier's transformation microscopy infrared spectroscopy). The contentof particles is based on the ratio of peak arising from a carbonyl groupin a polyester to peak arising from materials other than the polyester.In order to convert the peak height ratio to the corresponding weightratio, the ratio of polyester weight to a total weight of polyester plusother materials is determined from a calibration curve prepared inadvance with use of samples of known weights. An X-ray microanalyzer mayalso be employed when found necessary. In the case where a solvent canbe selectively used which dissolves a polyester, but does not dissolve aparticle material, the polyester is dissolved, and the particle materialis separated centrifugal from the polyester. Thus, the weight percentageof the particle material is determined.

Additionally, the content of particles in a surface zone of a test filmis determined as follows; that is, the film is slit into the form of atape of ½ inch in width and then brought into perpendicularly intimatecontact with a single-edged knife on a surface of the tape wherepolyester A has been laminated. With the knife edge forced into the tapeby 0.5 mm; the tape is then traveled at a distance of 20 cm (travelingtension: 500 g and traveling speed: 6.7 cm/second). Subsequentmeasurement is made, by means of the above method of determining thecontent of particles, of the content of particles in suchmatter ashaving been rubbed off the tape surface and attached to the knife edge.

(4) Heat Shrinkage

A 15 cm long, 1 cm wide film is placed without its ends fixed andmeasured as to its dimensional changes in the lengthwise and widthwisedirections after standing for 30 minutes at 80° C. If a dimensionalchange is small, but if a measurement accuracy of 0.1% or below isnecessary, then enlargement is done by a universal projector. A largenumerical value in either one of the lengthwise and widthwise directionsis taken as the heat shrinkage of the film.

(5) Surface Roughness Ra, 10-Point Average Roughness Rz andProtrusion-Protrusion Spacing Sm

The surface roughness Ra, 10-point average roughness Rz andprotrusion-protrusion spacing Sm of a film are measured by the use of ahigh-precision film-flatness measuring device, ET-10, manufactured byKosaka Laboratories. The measuring conditions are given below, and 20cycles of measurement are conducted while the film is being scannedwidthwise, after which the resultant numerical values are averaged.

radius of feeler tip: 0.5 μm

load of feeler: 5 mg

length of measurement: 1 mm

cut-off value: 0.08 mm

The definitions of Ra, Rz, Sm and the like are disclosed for instance in“Methods for Measurement and Evaluation of Surface Roughness” edited byJiro NARA (General Technical Center, 1983).

(6) Thickness of Film Laminate

A laminate film is observed cross-sectionally at an accelerating voltageof 100 kV with use of a transmission electron microscope (H-600Typemanufactured by Hitachi Ltd.) and by means of a ultra slicing method(Ru04 dyeing). The interface of the laminate is captured, from which thethickness of the laminate is determined. Magnifications are notparticularly restricted since they are usually chosen depending on thethickness of laminates to be measured. However, 1 tens of thousands-10tens of thousands are suitable.

In the alternative, a depth distribution of particle concentrations isdetermined by means of a secondary ion qualitative analyzer, X-rayphotoelectron spectroscopy, infrared spectroscopy or a constant focalmicroscope. The maximum value in the direction of depth is determined onthe basis of the particle surface, and a depth found equivalent to ½ ofthe maximum value is taken as the thickness of the laminate.

(7) Crystallization Parameter ΔTcg

A film is slit into the form of a ½ inch wide tape and then brought intoperpendicularly intimate contact with a single-edged knife. With theknife edge forced into the tape by 0.5 mm, the tape was then traveled ata distance of 20 cm (traveling tension: 500 g and traveling speed: 6.7cm/second). Matter rubbed off the tape surface and attached to the knifeedge was collected in an amount of 10 mg, which matter is used as asample. When rubbed matter comes short of 10 mg in single traveling,another fresh film is treated in the same manner as mentioned above inorder to prepare a total of 10 mg of a sample.

Measurement is made with DSC (differential scanning calorimeter). Anamount of 10 mg of the sample is set in a DSC device and melted at 300°C. for 5 minutes, followed by quenching of the melt in liquid nitrogen.The resultant specimen is heated at 10° C./minute and checked in respectof its glass transition point Tg. Temperature rise is continued, and acrystallizing exothermic peak temperature derived from a glass state istaken as a cold crystallization temperature Tcc, and an endothermic peaktemperature derived from crystal fusion is taken as a fusion temperatureTm. Likewise, a crystallizing exothermic peak temperature derived fromduring temperature drop is taken as a crystallization temperature intemperature drop Tmc. The difference between Tcc and Tg (Tcc−Tg) isdefined as the crystallization index ΔTcg.

(8) Oligomer Preclusion

A film is allowed to stand in an oven at 150° C. for 30 minutes, therebyforcibly separating oligomeric matter on the film surface. After beingdeposited with aluminum, the resultant film surface is photographed by adifferential interference microscope at an overall magnification of 400times. Observation is made in 25 fields on the photograph. The number ofoligomers is counted at each of the fields, and the total number istaken as the number of surface-separated oligomers (piece/mm²). Lessthan 80 pieces/mm² in number and smaller than 1 mm in size on thephotograph are adjudged to be excellent, and more than 80 pieces/mm² butless than 100 pieces/mm²in number and smaller than 1 mm in size areadjudged to be good. More than 100 pieces/mm² in number, or larger than1.5 mm in size is adjudged to be bad.

(9) Wear Resistance

A film slit to a width of ½ inch is allowed to travel on a guide pin(surface roughness Ra 100 nm) with use of a tape traveling tester(traveling speed 500 m/minute, traveling cycle 1, winding angle 60° andtraveling tension 30 g). Scratches on the film is microscopicallyexamined. Less than 3 flaws of 2.5 μm or above per tape width isadjudged to be excellent. Less than 10 flaws is good and more than 10flaws bad.

In the case of a film of more than 30 μm in overall thickness, travelingis effected on a guide pin (surface roughness Ra 100 nm) with a tapetraveling tester in the same manner as is done above, but except thattraveling speed 2 m/minute, winding angle 90° and traveling tension 200g are used. Evaluation is made by like judgements.

(10) Modulus

JIS K-7127 is followed. Measurement is made at 25° C. and at 65% RH bythe use of a tensile tester manufactured by Toyo Instruments Co. Asample is cut to a 10 mm wide, 200 mm long strip in the direction ofmeasurement, and the chuck-to-chuck space at initial tensile is 100 mmand tensile speed 300 mm/minute.

(11) Adhesiveness

As an ultraviolet-curable ink, a FLASH DRY FD-OL black (manufactured byToyo Ink Manufacturing Co., Ltd.), is used and coated on a film in athickness of 2 μm by means of roll coating. Subsequently, theultraviolet-curable ink is cured by irradiation with an ultraviolet lamp(80 W/cm and 5 seconds)

Adhesiveness is evaluated by cross-cutting the ink-cured film at anumber of 100 in an area of 1 mm² and by bonding a cellophane tape overthe cross-cut film and pressing the tape against the tape with use of arubber roll (3 strokes at a load of 19.6 N), followed by releasing ofthe tape at an angle of 90 degrees. A mode of 4-grade evaluation is used(⊙: 100, ◯: 80-99, Δ: 50-79 and ×: 0-49).

(12) Haze

Haze is measured by use of a full-automatic direct-reading hazecomputer, HGM-2DP, (for C light source) (Suga Instruments Co., Ltd.).Evaluation is made by the average of 10-point measurements.

haze: H (%)=(Td/Tt )×100

Td (%)=[{T4−T3×(T2/T1)}/T1]×100 (diffused transmission)

Tt (%)=(T3/T1)×100 (transmission of all light rays)

(T1: incident light, T2: all transmitted light, diffused light of

device T4: diffused transmitted light)

(13) Haze after Forced Heating

A film to be tested is fixed to a metal frame with binding clips andallowed to stand in a hot-air oven at 80° C. for 3 days. The haze ofthis film is determined by the method itemized above as (1).

(14) Output Characteristics (C/N)

Over a film according to the present invention is disposed, in thepresence of a trace of oxygen, a deposited layer of a cobalt-nickelalloy (Ni 20% by weight) in a thickness of 200 nm. A carbon-protectivefilm is further formed on the deposited surface in conventional fashionand then slit to a width of 8 mm, whereby a pan cake is prepared. Next,the pan cake is assembled in a length of 200 m into a cassette so as toprovide a cassette tape.

The resultant cassette tape is applied to a commercially available VTRdevice for Hi 8 (EV-BS3000 manufactured by Sony Corporation). C/N ismeasured at 7 MHz±1 MHz.

The C/N value thus obtained is compared to that of a commercial cassettetape (120-minute ME) for Hi 8 and evaluated as follows:

+more than 3 dB: excellent

+1-+3 dB: good

+less than 1 dB: bad

When being higher in a range of more than +1 dB than those of acommercial video tape (120-minute ME) for Hi 8, the outputcharacteristics are acceptably useful for VTR tapes of a digitalrecording system.

(15) Wear Resistance and Friction Coefficient

A film is slit into the form of a tape of ½ inch in width and caused totravel on a stainless steel-made guide pin (surface roughness: 100 nm byRa) with use of a tape traveling tester (traveling speed: 250 m/minute,winding angle: 60°, inlet-side tension 50 g and traveling cycle 1).

Initial μk is determined by the following equation.

μk=3/π1n(T/50)

where T denotes the tension on an outlet side. Less than 0.3 in μk isjudged to be acceptably slidable and more than 0.3 unacceptablyslidable. The μk value of 0.3 is a critical point at whichinconveniences would be liable to occur due to inadequate slidability atworking steps, for example, at a printing step.

EXAMPLES

With reference to the following examples, the present invention will nowbe described in relation to its embodiments.

Example 1

PPT was produced by ester exchange reaction and polycondensationreaction of dimethyl terephthalate and 1,3-propanediol.

PPT pellets were vacuum-dried (3 Torr) at 120° C. for 8 hours, and PETpellets were vacuum-dried (3 Torr) at 180° C. for 8 hours. Polymer A: aPPT polymer and polymer B: a PET polymer containing 0.1% by weight ofcalcium carbonate particles of 0.8 μm in particle diameter were put inan extruder 1 and an extruder 2, respectively, and melted at 265° C. andat 280° C., respectively. After being filtered with high precision, bothof the polymers were laminated at a rectangular intermixing portion intothree-layered formation (A/B/A).

By use of electrostatic casting, the resultant laminate was wound arounda casting drum of 20° C. in surface temperature so that the laminate wascooled and solidified to prepare a non-stretched film. In this instance,the ratio of gap of cap slit/ thickness of non-stretched film was set at10. Moreover, the discharge of each of the extruders was controlled toadjust the overall thickness of the non-stretched film and the thicknessof the associated layer A.

This non-stretched was stretched 3.5 times in a lengthwise direction andat a temperature of 96° C. Stretching was effected at four stages, eachstage using two pairs of rolls worked at varying peripheral speeds. Thefilm so stretched monoaxially was stretched 3.6 times in a widthwisedirection, at a temperature of 100° C. and with use of a tenter,followed by heat treatment under constant tension at 220° C. for 3seconds. In that way, a biaxially oriented film was obtained which hadan overall thickness of 6.3 μm and provided with a layer A of 0.3 μm inthickness. The characteristics of this biaxially oriented polyester filmare shown in Table 1, and the wear resistance has been found to be good.

Example 2

A biaxially oriented polyester film was produced with use of the samepolymer A as used in Example 1, PET changed to contain an amount of 0.1%by weight of divinyl benzene particles of 0.8 μm in particle diameter,and the laminate thickness changed to be at 1.0 μm. The characteristicsof this polyester film are shown in Table 1, and the wear resistance hasbeen found to be good.

Example 3

The procedure of Example 1 was repeated except that the thickness of thelayer A of a PPT polymer was changed to 0.05 μm, whereby a biaxiallyoriented polyester film was provided. The characteristics of thispolyester film are shown in Table 1, and the wear resistance has beenfound to be good.

Comparative Example 1

A biaxially oriented polyester film was provided by using thosematerials used in Example 1, but by changing the film structure,laminate thickness, stretching conditions and the like. Thecharacteristics of this polyester film are shown in Table 1, and thewear resistance has been proved to be bad.

Comparative Example 2

A biaxially oriented polyester film was obtained by using the polymer Aof Example 1 for use in the layer A, and a substantially particle-freePET polymer for use in the layer B, but by changing the laminatethickness, stretching conditions and the like. The characteristics ofthis polyester film are shown in Table 1, and the wear resistance hasbeen proved to be bad.

Comparative Example 3

A biaxially oriented polyester film was obtained by use of the polymer Bof Example 1, which film was of a single-layered structure and was 10 μmin thickness. The characteristics of this polyester film are shown inTable 1, and the wear resistance has been proved to be bad.

TABLE 1 Laminate Heat portion shrinkage Film structure Wear (layer A)(80° C., 30 min) Thickness of resis- polymer (%) layer A (μm) tanceExample 1 Polypropylene 0.31 A/B/A Good terephthalate 0.3 Example 2Polypropylene 0.35 A/B/A Good terephthalate 1.0 Example 3 Polypropylene0.29 A/B/A Good terephthalate 0.05 Comparative Polypropylene 0.85 A/BBad Example 1 terephthalate 4 Comparative Polyethylene 0.42 A/B/A BadExample 2 terephthalate 0.8 Comparative Polyethylene 0.35 Single layerBad Example 3 terephthalate 10

Example 4

PPT was produced by ester exchange reaction and polycondensationreaction of dimethyl terephthalate and 1,3-propanediol. Then, an aqueousslurry of aluminum silicate particles was prepared, which particles weresynthesized by reacting sodium silicate and sodium aluminate in anaqueous system by a wet method and had an aluminum ratio of 20% byweight in terms of aluminum oxide. This aqueous slurry was mixed withand kneaded in PPT pellets.

The particle-containing PPT pellets were mixed with substantiallyparticle-free PPT pellets in their respective appropriate amounts andthen vacuum-dried (3 Torr) at 120° C. for 8 hours. Polymer A: a PPTpolymer containing 0.2% by weight of aluminum silicate particles of 0.17μm in particle diameter and polymer B: a PET polymer containing 0.1% byweight of calcium carbonate particles of 0.8 μm in particle diameterwere put in an extruder 1 and an extruder 2, respectively, and melted at260° C. and at 280° C., respectively. After being filtered with highprecision, both of the polymers were laminated at a rectangularintermixing portion into two-layered formation (A/B).

With use of static casting, the resultant laminate was wound around acasting drum having a surface temperature of 20° C., whereby thelaminate was cooled and solidified to form a non-stretched film. In thisinstance, the ratio of gap of cap slit/thickness of non-stretched filmwas set at 10. Further, the discharge of each of the extruders wascontrolled to adjust the overall thickness of the non-stretched film andthe thickness of the associated layer A.

This non-stretched film was stretched 3.5 times in a lengthwisedirection and at a temperature of 93° C. Stretching was effected atthree stages, each stage using two pairs of rolls worked at varyingperipheral speeds. The film so stretched monoaxially was stretched 4.8times in a widthwise direction, at a temperature of 95° C. and with useof a tenter, followed by heat treatment under constant tension at 220°C. for 3 seconds and subsequent treatment in an intermediate coolingzone at 120° C. for 7 seconds. Thus, a biaxially oriented polyester filmwas obtained which had an overall thickness of 11 μm and provided with alayer A of 0.3 μm thick layer A. The characteristics of this biaxiallyoriented polyester film are shown in Table 2, and the wear resistanceand oligomer preclusion have been found excellent.

Examples 5 and 6 and Comparative Examples 4 and 5

Biaxially oriented polyester films were produced in the same manner asin Example 4, but with the use of varied kinds, particle diameters andcontents of particles, varied thickness of laminates and the like. InExample 6, polymer C for use in a layer C was the particle-free PPTpolymer as in Example 1, and the laminate thickness was 1 μm. As isclear from Table 2, the films of Examples 5 and 6 are excellent inrespect of wear resistance and oligomer preclusion. The films ofComparative Examples 4 and 5 are not Good as Regards TheseCharacteristics

Examples 7-9

Biaxially oriented polyester films each of 7 μm in overall thicknesswere provided in which the PPT polymer of Example 1 had been used aspolymer A, polymer B had been composed of a PET polymer containing 0.1%by weight of crosslinkable divinyl benzene particles of 0.6 μm inaverage particle diameter, and changes had been made to the particlediameters, contents and contents of particles, thickness of laminates,stretching conditions and the like. As evidenced from the results ofTable 2, the biaxially oriented polyester films of the present inventionhave been excellent in respect of wear resistance and oligomerpreclusion.

TABLE 2 Polymer of Particle Young's modulus laminate Particle diameterHeat shrinkage lengthwise/ Film structure portion (μm) (80° C., 30 min)widthwise thickness of Wear Oligomer (layer A) Content (wt %) (%)(ratio) layer A (μm) resistance preclusion Example 4 PolypropyleneAluminum silicate 0.31 4.5/4.8 A/B Excellent Excellent terephthalate0.17 (0.93) 0.3 0.2 Example 5 Polypropylene Calcium carbonate 0.354.8/4.9 A/B/A Excellent Excellent terephthalate 0.8 (0.97) 1.0 0.1Example 6 Polypropylene Aluminum silicate 0.36 5.2/5.2 A/B/C ExcellentExcellent terephthalate 0.03 (1.0) 0.05 0.7 Example 7 PolypropyleneAbsent 0.36 5.2/4.2 A/B/A Excellent Excellent terephthalate (1.23) 0.1Example 8 Polypropylene Absent 0.32 4.9/5.6 A/B/A Excellent Excellentterephthalate (0.88) 0.5 Example 9 Polypropylene Absent 0.38 5.5/6.5A/B/A Excellent Excellent terephthalate (0.85) 0.3 ComparativePolypropylene Aluminum silicate 0.82 4.8/7.2 A/B Bad Excellent Example 4terephthalate 0.17 (0.67) 3 0.005 Comparative Polyethylene Silica 0.424.3/6.2 A/B/A Bad Bad Example 5 terephthalate 0.5 (0.69) 0.8 0.5

Example 10

PET pellets 1 (IV 0.72) were derived from polymerization in conventionalmanner and dried in vacuum (3 Torr) at 185° C. for 3hours. Pellets 2(IV0.95) were also prepared by ester exchange reaction andpolycondensation reaction of dimethyl terephthalate and 1,3-propanedioland dried in vacuum (3 Torr) at 140° C. for 3 hours. Additionally,pellets 3 were prepared by polymerizing PET with an inert particlematerial (average particle diameter: colloidal silica particles of 0.25μm in average particle diameter, added in a content of 0.5% by weightduring polymerization, and IV 0.65) and dried in vacuum (3 Torr) at 185°C. for 3 hours.

Upon drying of each of the pellets 1 and the pellets 2, 19.7% by weightof the particle-free PET pellets 1, 80% by weight of the PPT pellets 2and 0.3% by weight of the particle-containing PET pellets 3 were mixedto prepare polyester A, and the PET pellets 1 were used as polyester B.The two polyesters were supplied to two extruders. Polyester A wasmelted in the extruder 1 at 265° C., while polyester B was melted in theextruder 2 at 290° C. Both melts were laminated together in arectangular intermixing block (feed block) for use in three-layeredlamination and, by means of electrostatic casting, was caused to windaround a casting drum of 22° C. in surface temperature and to cool andsolidify as it was. Thus, a non-stretched laminated film was formedwhich was of a three-layered structure of A/B/A. Prior to stretching,this non-stretched film was preheated by passage through four siliconerolls each of 85° C. in surface temperature. In addition, the film sopreheated was stretched 3.2 times in a lengthwise direction at 95° C.,then stretched 4.0 times in a widthwise direction at 95° C. with use ofa known tenter and again stretched 1.3 times in the lengthwise directionat 90° C. Heat treatment was subsequently effected under constanttension at 220° C. for 5 seconds, followed by treatment in anintermediate cooling zone at 120° C. for 7 seconds. Heat treatment wasonce again conducted in a relaxation ratio of 2% at 100° C. for 3seconds. Thus, a biaxially oriented polyester film was provided whichhad a lamination thickness of 1 μm and an overall thickness of 12 μm.

Examples 11 and 12

Biaxially oriented polyester films were provided in the same manner asdone in Example 10 and with the use of polymers for layers A formulatedin those ratios shown in Table 3. These polyester films were formed withvaried particle diameters, extrusion melting temperatures, stretchingtemperatures and the like. In Example 11, stretching was effected 4.5times in a lengthwise direction at four separate stages and 4.0 times ina widthwise direction. In Example 12, stretching was effected 4.8 timesin a lengthwise direction at four separate stages and 4.0 times in awidthwise direction.

Comparative Examples 6-8

As polyester A, use was made of those prepared by mixing PET pellets 1,PPT pellets 2 and particle-containing PET pellets 3 in those ratiosshown in Table 3. In the same manner as done in Example 11 but withstretching temperatures and stretching percentages varied, biaxiallyoriented laminated polyester films were formed which had an overallthickness of 12 μm (thickness on one side of layer A: 1 μm and 3.5 μm inComparative Example 8).

Performance evaluation was made of the films prepared in Examples 10-12and Comparative Examples 6-8 with the results tabulated in Table 3. Allthe samples within the scope of the present invention have beenexcellent in oligomer preclusion and dimensional stability as comparedto the comparative samples.

Example 13

PPT was derived from ester exchange reaction and polycondensation ofdimethyl terephthalate and 1,3-propanediol.

Upon drying of PPT pellets in vacuum (3 Torr) at 120° C. for 8 hours,polymer A: a PPT polymer and polymer B: a substantially particle-freepolyethylene terephthalate polymer were put in an extruder 1 and anextruder 2, respectively, and melted at 260° C. and at 280° C.,respectively. After being filtered with high precision, the two polymerswere laminated at a rectangular intermixing portion into three-layeredarrangement (A/B/A)

By means of electrostatic casting, the resultant laminate was woundaround a casting drum of 20° C. in surface temperature, and cooled andsolidified as it was, whereby a non-stretched film was formed. In suchinstance, the ratio of gap of cap slit/thickness of non-stretched filmwas set to be 10. The discharge of each of the extruders was alsocontrolled to adjust the overall thickness of the film and the thicknessof the layer A.

This non-stretched film was heat-treated on silicone rolls at a treatingtemperature of 85° C. for a period of time of 10 seconds and thenstretched 3.3 times in a lengthwise direction at a stretchingtemperature of 93° C. and at a stretching speed of 10000%/minute.Stretching was effected at three stages, each stage using two pairs ofrolls worked at varying peripheral speeds. The film so monoaxiallystretched was stretched 3.5 times in a widthwise direction at 96° C.with use of a tenter and further stretched 1.1 times in the widthwisedirection at 95° C. Heat treatment was then effected under constanttension at 220° C. for 3 seconds, followed by treatment in anintermediate cooling zone at 120° C. for 7 seconds. Heat treatment wasonce again conducted with a relaxation ratio of 2% at 100° C. for 3seconds. Thus, a biaxially oriented polyester film was provided whichhad an overall thickness of 5 μm and a thickness of 1.0 μm in the layerA. The characteristics of this polyester film are shown in Table 4, andthe wear resistance has been found excellent.

TABLE 3 Lengthwise stretch- Lengthwise Composition of ing condition pre-Young's Heat shrinkage S layer A (wt %) Film structureheating/stretching modulus E (80° C., 30 min) Oligomer dimensional PETPPT Particle (μm) temperature (° C.) (Gpa) (%) 0.08E−S preclusionstability Example 10 19.7 80 Silica A/B/A 85/95 6.3 0.41 0.094 GoodExcellent 0.3 1/10/1 Example 11  9.75 90 Silica A/B/A 80/85 5.6 0.360.088 Excellent Excellent 0.25 1/10/1 Example 12 19.75 80 Silica A/B/A85/90 5.8 0.37 0.094 Good Excellent 0.25 1/10/1 Comparative 99.8 AbsentSilica A/B/A 85/93 5.2 0.35 0.066 Bad Bad Example 6 0.2 1/10/1Comparative 59.8 40 Silica A/B/A 70/85 4.1 0.29 0.038 Bad Bad Example 70.2 1/10/1 Comparative 10 90 Absent A/B/A 80/90 5.5 0.82 −0.38 ExcellentBad Example 8 3.5/5/3.5

TABLE 4 Lengthwise stretching (° C.) Heat shrinkage Average roughness Ra(nm) Film structure Laminate portion Preheating temp. (80° C., 30 min)Rz/Ra Thickness of Wear (layer A) polymer Stretching temp. (%)Protrusion spacing Sm (μm) layer A (μm) resistance Example 13Polypropylene 85 0.35 25 A/B/A Excellent terephthalate 90  8.5  1.0 (noparticle added)  9.0 Example 14 Polypropylene 90 0.31 30 A/B Excellentterephthalate 95  9.0  0.5 (no particle added)  9.2 Example 15Polypropylene 95 0.33 20 A/B/A Excellent terephthalate 93  8.0  0.2 (noparticle added)  8.2 Comparative Polypropylene 70 0.54  4 A/B/A BadExample 9 terephthalate/ 90 15  1.0 Polyethylene 40 terephthalate(50/50) (no particle added) Comparative Polyethylene 85 0.33 15 A/B/ABad Example 10 terephthalate 90 16  2.0 (particle diameter: 12 0.8 μmcalcium carbonate: 0.1 wt %) Comparative Polyethylene 90 0.35 16 Singlelayer Bad Example 11 terephthalate 95 20 10 (particle diameter: 35 0.8μm calcium carbonate: 0.1 wt %)

Examples 14 and 15 and Comparative Examples 9-11

Biaxially oriented polyester films were provided in the same manner asdone in Example 1 but with PPT laminating thickness, lengthwisestretching temperatures and the like made variable. As is evident fromTable 4, the polyester films within the scope of the present inventionare excellent in wear resistance, whereas the other comparative filmsare unacceptable in that respect.

Example 16

PPT was derived from ester exchange reaction and polycondensationreaction of dimethyl terephthalate and 1,3-propanediol.

Upon drying of PPT pellets in vacuum (3 Torr) at 120° C. for 8 hours,polymer A: a PPT polymer and polymer B: a PET polymer containing 0.1% byweight of calcium carbonate particles of 0.8 μm in particle diameterwere charged in an extruder 1 and in an extruder 2, respectively, andmelted at 260° C. and at 280° C., respectively. After being filteredwith high precision, the two melts were laminated at a rectangularintermixing portion into three-layered formation (A/B/A).

By means of electrostatic casting, the resultant laminate was woundaround a casting drum of 20° C. in surface temperature, and cooledsolidified as it was. Thus, a non-stretched film was formed. In suchinstance, the ratio of gap of cap slit/thickness of non-stretched filmwas set at 10. The discharge of each of the extruders was alsocontrolled to adjust the overall thickness of the film and the thicknessof the layer A.

The non-stretched film thus formed was stretched 3.5 times in alengthwise direction at a preheating temperature of 85° and at astretching temperature of 90° C. Stretching was effected at threestages, each stage using two pairs of rolls worked at varying peripheralspeeds. The film so monoaxially stretched was stretched 3.5 times in awidthwise direction at 100° C. with use of a tenter, followed by heattreatment under constant tension at 220° C. for 10 seconds, whereby abiaxially oriented film was provided with an overall thickness of 60 μmand a thickness of 1.0 μm in the layer A. The characteristics of thisbiaxially oriented polyester film are shown in Table 5, and the wearresistance has been found excellent.

Examples 17 and 18 and Comparative Example 12

Biaxially oriented polyester films were obtained in the same manner asdone in Example 16 but with polymer types in laminated portions,laminating thickness, lengthwise stretching temperatures and the likemade variable. As is apparent from Table 5, the biaxially orientedpolyester films of Examples 17 and 18 exhibit excellent wear resistance,but that of Comparative Example 12 reveals inadequate wear resistance.

TABLE 5 Layer A Heat shrinkage Crystallization Laminate portion (80° C.,30 min) parameter Film structure Wear (layer A) polymer (%) ΔTcg (° C.)Thickness (μm) resistance Example 16 Polypropylene 0.28 30 A/B/AExcellent terephthalate 1/58/1 Example 17 Polypropylene 0.32 33 A/BExcellent terephthalate 2/58 Example 18 Polypropylene 0.25 33 A/B/CExcellent terephthalate 0.5/58.5/1 Comparative Polypropylene 0.83 32A/B/A Bad Example 12 terephthalate 10/25/10

Example 19

As thermoplastic polymers B and C, a PET polymer was used which resultedfrom a conventional mode of polymerization and did not substantiallycontain a particle material. For a polymer layer A, a PPT polymer (IV0.93) was used which did not contain a particle material. The twopolymers were dried for 3 hours, respectively, at 180° C. and at 120° C.With use of three known extruders, melt extrusion was effected at 260°C. (polymer layer A), at 280° C. (polymer layer B) and at 290° C.(polymer layer C) The melts were laminated together at a rectangularintermixing block (feed block) for use in three-layered lamination, andby means of electrostatic casting, the laminate was caused to windaround a metallic casting drum of 20° C. in surface temperature so thatthe laminate was cooled and solidified. Thus, a non-stretched film wasformed which was of a five-layered structure of B/A/C/A/B.

This non-stretched film was heat-treated on silicone rolls at a treatingtemperature of 140° C. for a period of time of 5 seconds and, whilebeing passed through the rolls, was stretched 3.8 times in a lengthwisedirection at four or separate stages at a stretching temperature of 95°C. and at a stretching speed of 10000%/minute and further stretched 5.2times in a widthwise direction at a stretching temperature of 100° C andat a stretching speed of 5000%/minute with use of a conventional tenter.Heat treatment was then conducted under constant tension at 220° C. for3 seconds, followed by treatment in an intermediate cooling zone at 120°C. for 7 seconds. Thus, a biaxially oriented laminated film was providedwhich had an overall thickness of 7 μm, a thickness of 0.05 μm in thelayer B and a thickness of 0.5 μm in the PPT laminated layer.

Examples 20 and 21

Biaxially oriented polyester films of a five-layered laminated structurewere formed in the same manner as in Example 19 but with the use ofvarying laminating thickness of layers A and B and stretchingconditions.

Example 22

The same manner as in Example 19 was followed. As the thermoplasticpolymer C, a PET recycling polymer (containing 0.05 wt. % of calciumcarbonate particles of 0.6 μm in particle diameter and 0.3 wt. % ofcolloidal silica particles of 0.3 μm in particle diameter) was put touse. On both sides was laminated, in a thickness of 0.8 μm, PPT of 1.0in IV as a polymer for a layer A, and for a layer B, polymer pellets ofPET were also prepared which contained 2 wt. % of δ type aluminaparticles of 20 nm in primary particle diameter. These pellets werediluted with the particle-free polymer pellets of PET used in Example 1such that the concentration of particles was set at 0.3 wt. % in thelayer B. Thus, a biaxially oriented laminated film was produced whichwas of a five-layered structure with an overall thickness of 7 μm.

TABLE 6 Thickness Thickness Thickness Surface TD Young's Laminate oflayer A of layer B of layer C Ta/Tb roughness Heat shrinkage modulusWear Oligomer structure (μm) (μm) (μm) (%) (nm) (80° C., 30 min) (Gpa)resistance preclusion Example 19 B/A/C/A/B 0.5 0.05 5.9 0.1 20 0.52 5.5Good Good Example 20 B/A/C/A/B 0.5 0.03 5.9 0.06 23 0.53 5.8 GoodExcellent Example 21 B/A/C/A/B 0.8 0.03 5.3 0.04 25 0.41 5.0 GoodExcellent Example 22 B/A/C/A/B 0.8 0.03 5.3 0.04 24 0.61 6.5 GoodExcellent Example 23 B/A/C 0.5 0.05 4.5 0.1 28 0.46 5.6 Good Good

Example 23

Polymer pellets of PET were prepared which contained 2 wt. % of siliconparticles having an average particle diameter of 0.8 μm. These pelletswere diluted with particle-free polymer pellets such that the content ina layer C was set to be 0.3 wt. %, whereby polymer pellets were obtainedfor use as a thermoplastic polymer C. For a layer A, use was made of thesubstantially particle-free PPT (IV 0.93) employed in Example 1, and fora layer B, polymer pellets were used in which alumina were contained(0.3 wt. %). With use of three extruders to provide B/A/C formation, abiaxially oriented laminated film was produced which was ofthree-layered structure having an overall thickness of 5 μm and providedwith a 0.5 μm thick layer A and a 0.05 μm thick layer B.

The characteristics of the films obtained in Examples 19-23 are shown inTable 6. All these films are excellent in regard to oligomer preclusionand abrasion resistance.

Examples 24-26 and Comparative Examples 13 and 14

A PET polymer, a PPT polymer and a polymer blend of PPT with PET (90:10)were used, all of which had been derived in known manner. In the casewhere particles were added, polyethylene terephthalate or polypropyleneterephthalate were used which were obtainable by use of a conventionalmode of polymerization and by use of particle-containing ethylene glycolor 1,3-propylene glycol.

The starting pellets each were dried for 3 hours at their respectivesuitable temperatures in the range of 120-180° C. With use of threeknown extruders, melt extrusion was effected at 260° C., at 280° C. andat 265° C., respectively, and the melts were laminated at a rectangularintermixing block (feed block) for use in three-layered lamination insuch a manner that those laminate structures tabulated in Table 7 couldbe attained. The resultant laminate was caused, by means ofelectrostatic casting, to wind around a metallic casting drum of 20° C.in surface temperature and to cool and solidify as it was. Thus, anon-stretched film was formed.

This non-stretched film was heat-treated at 85° C. on conventionalsilicone rubber rolls and, while being passed through the rolls, wasstretched 3.8 times in a lengthwise direction at three or more stages,at a stretching speed of 20000%/minute and at a stretching temperatureof 93° C. and further stretched 3.8 times in a widthwise direction at100° C. with use of a known tenter. When it was found necessary,widthwise stretching was conducted once again, followed by heattreatment under constant tension at 220° C. for 3 seconds and bysubsequent treatment in an intermediate cooling zone at 120° C. for 7seconds. Thus, biaxially oriented laminated films were produced withthose thickness shown in Table 7.

A metal deposit was disposed on the surface of a third layer. In theExamples, the films have been found attainable with adequate centerlineaverage roughness Ra and desirable protrusion-protrusion spacing Sm andhence with excellent output characteristics and traveling capability.The films of Comparative Examples 13 and 14 are inferior in thoseproperties.

TABLE 7 Film structure Heat shrinkage 1^(st) layer 3^(rd) layer LaminatePolymer of laminate portion (80° C., 30 min) Average roughness Ra (nm)Ra (nm) thickness Traveling Output 1^(st) layer/2^(nd) layer/3^(rd)layer (%) Protrusion spacing Sm (μm) Sm (μm) (μm) capabilitycharacteristic Example 24 PPT/PET/PP · PET (90:10) 0.35 25  5 A/B/C GoodGood  9.0 13 0.5/5/0.5 Example 25 PPT/PET/PPT 0.31 23  7 A/B/A ExcellentExcellent  9.2 12 1/4.8/0.2 Example 26 PPT/PET/PPT 0.53 20  7 A/B/AExcellent Good 1^(st) layer: silica: 0.8 μm  9.2 13 0.8/4.4/0.8 content:0.02 wt % Comparative PPT/PET 0.58  5 16 A/B Good Bad Example 13 2^(nd)layer: calcium 0.8 μm 16 18 0.2/5.8 carbonate: content: 0.02 wt %Comparative PPT/PET/PPT 0.39 18 18 A/B/A Good Bad Example 14  9.6  9.61/4/1

Example 27

Polymer A: PET containing 0.015% by weight of colloidal silica of 0.4 μmin average particle diameter and 0.005% by weight of colloidal silica of1.5 μm in average particle diameter, and polymer B: PPT weresufficiently dried in vacuum. Polymer A was supplied to an extruder 1and polymer B to an extruder 2, and melted at 280° C. and at 260° C.,respectively. After being filtered with high precision, the two polymerswere melt-extruded at a rectangular intermixing portion into atwo-layered laminated structure. The resultant laminate was caused, bymeans of electrostatic casting, to wind around a casting drum of 22° C.in surface temperature and to cool and solidify as it was, whereby anon-stretched film was formed. This non-stretched film was stretched 3.5times in a lengthwise direction at a temperature of 95° C. The resultantfilm was subjected to atmospheric corona discharging on one side thereofwhere PPT had been disposed so that an adhesive layer-forming coatingsolution a was coated over the corona-discharged surface. Themonoaxially stretched film thus coated was introduced, while beingclamped with clips, in a preheating zone where it was heated at 110° C.,and successively continuously stretched 3.5 times in a widthwisedirection in that preheating zone. Heat treatment was then effected at225° C., followed by treatment in an intermediate cooling zone at 120°C. for 7 seconds. Thus, a highly adhesive polyester film was produced asdesired.

In such polyester film, the substrate film of PET was 49.5 μm thick, thePPT layer 0.5 μm and the adhesive layer 0. 15 μm.

The results obtained are shown in Table 8. The haze after forced heatinghas been prevented from being increased, and the adhesiveness isexcellent.

Adhesive layer-forming coating solution a: aqueous coating solutioncomposed of polyester resins (2 kinds) consisting of those componentsindicated below and glycol components, and a crosslinking agent.

polyester resin A 50 parts by weight acid components terephthalic acid50 mol % isophthalic acid 25 mol % sebacic acid 24 mol % 5-sodiumsulfonyl itaconate 1 mol % glycol components ethylene glycol 55 mol %neopentyl glycol 45 mol % polyester resin B 50 parts by weight acidcomponents terephthalic acid 87.5 mol % 5-sodium sulfonyl 12.5 mol %isophthalate glycol component ethylene glycol 100 mol % methylolatedmelamine 5 parts by weight crosslinking agent

Those components were mixed to meet the ratios of solid contentsspecified above and diluted with water to a solid content concentrationof 5% by weight.

TABLE 8 PPT layer Haze (%) PPT Heat shrinkage Average roughness Ra (nm)Before After ratio Thickness (%) Rz/Ra Adhesive forced forced (wt %)(μm) (80° C., 30 min) Protrusion spacing Sm (μm) layer heating heatingAdhesiveness Example 27 100 0.5 0.29 35 a 1.0 1.4 ⊚  8.2 13

Industrial Applicability

The biaxially oriented polyester film according to the present inventionis excellent in wear resistance and oligomer preclusion, and therefore,is useful for magnetic recording media.

What is claimed is:
 1. A biaxially oriented polyester film characterizedin that said polyester film comprises at least one film layer composedpredominantly of polypropylene terephthalate and has a heat shrinkage of0.8% or below after standing at 80° C. for 30 minutes, wherein saidpolyester film has an X/Y ratio of 5 or above where X denoted the numberof protrusions defined on the surface of said propyleneterephthalate-predominated film layer, and Y denotes the number ofparticles contained in said propylene terephthalate-predominated filmlayer.
 2. A biaxially oriented polyester film according to claim 1,wherein said polypropylene terephthalate-predominated film layer has athickness of 0.01-3.0 μm.
 3. A biaxially oriented polyester filmaccording to claim 1 or 2, wherein said polypropyleneterephthalate-predominated film layer comprises 60-99.9% by weight ofpolypropylene terephthalate and 40-0.1% by weight of polyethyleneterephthalate.
 4. A biaxially oriented polyester film according to claim1 or 2, wherein said polypropylene terephthalate-predominated film layercontains 0.01-3% by weight of a particle material having an averageparticle diameter of 0.01-2.0 μm.
 5. A biaxially oriented polyester filmaccording to claim 4, wherein said polyester film meets the equation of0.2 d≦t≦10 d where t denotes the thickness (nm) of said polypropyleneterephthalate-predominated film layer, and d denotes the averageparticle diameter (nm) of said particle material contained in said filmlayer.
 6. A biaxially oriented polyester film comprising at least onefilm layer composed predominantly of polypropylene terephthalate, atleast one surface of said polypropylene terephthalate-predominated filmlayer having a surface roughness Ra of 5-120 nm, a 10-point averageroughness Ra/Rz of 12 or below and a protrusion-to-protrusion spacing Smof 15 μm or below.
 7. A biaxially oriented polyester film according toclaim 6, wherein said polypropylene terephthalate-predominated filmlayer has a thickness of 0.01-3.0 μm.
 8. A biaxially oriented polyesterfilm according to claim 6 or 7, wherein said polypropyleneterephthalate-predominated film layer comprises 60-99.9% by weight ofpolypropylene terephthalate and 40-0.1% by weight of polyethyleneterephthalate.
 9. A biaxially oriented polyester film according to claim6 or 7, wherein said polypropylene terephthalate-predominated film layercontains 0.01-3% by weight of a particle material having an averageparticle diameter of 0.01-2.0 μm.
 10. A biaxially oriented polyesterfilm according to claim 9, wherein said polyester film meets theequation of 0.2 d≦t≦10 d where t denotes the thickness (nm) of saidpolypropylene terephthalate-predominated film layer, and d denotes theaverage particle diameter (nm) of said particle material contained insaid propylene terephthalate-predominated film layer.
 11. A biaxiallyoriented polyester film according to claim 6, wherein said polyesterfilm has an X/Y ratio of 5 or above where X denoted the number ofprotrusions defined on the surface of said propyleneterephthalate-predominated film layer, and Y denotes the number ofparticles contained in said propylene terephthalate-predominated filmlayer.
 12. A biaxially oriented polyester film according to claim 1 or6, wherein said polyester film has a Young's modulus of 4.5 GPa or abovein the lengthwise and widthwise directions thereof.
 13. A biaxiallyoriented polyester film according to claim 1 or 6, wherein the ratio ofYoung' modulus in the lengthwise direction to Young's modulus in thewidthwise direction (lengthwise Young's modulus/widthwise Young'smodulus) is in the range of 0.7-1.5.
 14. A biaxially oriented polyesterfilm according to claim 1 or 6, wherein said polyester film meets theequation of 0.08 E−S≧0.08 where E denotes the modulus (GPa) in thelengthwise direction, and S denotes the heat shrinkage (%) in thelengthwise direction.
 15. A biaxially oriented laminated polyester filmaccording to claim 1 or 6, wherein said polyester film has an overallthickness of 50-100 μm.
 16. A biaxially oriented laminated polyesterfilm characterized in that said polyester film has a laminate structurecomprising three or more film layers and provided with two outermostfilm layers each formed of a film layer composed predominantly ofpolypropylene terephthalate, wherein one of said two outermost filmlayers has a centerline surface roughness Ra of 9 nm or below and asurface protrusion-to-protrusion spacing Sm of 15 μm or below, and theother outermost film layer has a centerline surface roughness between 9nm or above and 30 nm or below, a surface protrusion-to-protrusionspacing Sm of 15 μm or below and a heat shrinkage of 0.8% or below afterstanding at 80° C. for 30 minutes.
 17. A biaxially oriented laminatedpolyester film having a laminate structure of at least three layerscomprising a polymer layer composed of predominantly of polypropyleneterephthalate, layer A, a polymer layer comprised of a thermoplasticresin B, layer B and a polymer comprised of a thermoplastic resin C,layer C, wherein said layer A is disposed on at least one surface ofsaid layer C, said layer B is disposed on at least one surface of saidlayer A, said layer A has a thickness of less than 1 mm, and said layersA and B meet the equation: 0.01≦Tb/Ta<1 where Ta denotes the thicknessof said layer A, and Tb denotes the thickness of said layer B.
 18. Abiaxially oriented polyester film according to claim 17, wherein thethickness of said layer C is larger than 50% of the overall thickness ofsaid polyester film.
 19. A biaxially oriented polyester film accordingto claim 17 or 18, wherein said thermoplastic resin C is polyethyleneterephthalate.
 20. A biaxially oriented polyester film according to anyone of claims 1, 6, 16 and 17, wherein said polyester film furtherincludes an adhesive layer disposed on at least one surface thereof.